The present invention relates to mechanical drive assemblies, and in particular to those which are used to effect precise movement of a slit plate in scanning and imaging devices.
X-ray imaging devices generally employ a collimated beam of radiation which is directed from a focal spot through an object to be scanned, such as a live human or animal patient, or a package to be inspected. A bank of radiation-sensitive detectors is located opposite the focal spot, with the object to be scanned between the focal spot and the detectors. The focal spot and detectors may be fixed relative to one another on a gantry which rotates about the object to be scanned. The detectors receive radiation which has passed through the object and which has therefore been attenuated to varying degrees as. a function of the density of structures within the object and in the radiation path. The detectors generate signals which correspond to the detected density values, and these signals are used to map the object so that the internal structures can be seen.
The radiation beam is collimated by passage of the beam through a slit in a radiation-opaque plate. Typically the plate contains a number of slits of differing widths, so that the beam can be collimated to different widths. For convenience, the slit plate is generally moved into the desired position by translation in a direction transverse to the direction of the beam (generally referred to as the z-direction).
It is important that the slit through which the radiation beam passes to be collimated be located properly relative to the focal spot and the detectors so as to admit precisely as much radiation as can be detected by the detectors, for maximum data acquisition for each scan of the object. If the slit is not properly aligned with the focal spot and the detectors; some of the radiation pass through the object but may not be received by the detectors. Any radiation which passes through the object without being detected subjects the object to radiation exposure without providing useful imaging data, and this is undesirable.
It is also important that movement of the slit plate be smooth, without backlash, accurate and precise. This is especially critical in scanners which may rotate at speeds greater than one revolution per second.
In prior art scanners, the slit plate is typically mounted on a slide which is adapted for travel along a set of parallel shafts. The slide is driven by a drive mechanism, such as a precision leadscrew, which is mounted to a stepper motor. Misalignment of the shafts, of the slide on the shafts, and binding of the leadscrew, commonly occur because it is difficult to maintain these structures in precise alignment. Because each structure is independently mounted, alignment of all three is a costly and time-consuming undertaking and must be checked and repeated frequently.
It would therefore be an advancement in the art of precision drive assemblies to overcome the disadvantages of the prior art.
According to one aspect of the invention, there is provided a backlash-resistant sliding assembly for supporting a collimator. The assembly includes a base having an elongated aperture for passage of an x-ray beam therethrough, a first shaft secured to the base and extending generally perpendicular to the aperture of the base, a second shaft spaced from, and extending substantially parallel to the first shaft, the second shaft rotatably secured to the base and having a threaded portion and at least one non-threaded portion. The assembly also includes a carrier adapted to support a collimator having at least one elongated slit, the carrier including a first support member having a bore slidingly receiving the first shaft, a second support member having a bore receiving the non-threaded portion of the second shaft, and an elongated opening extending between the support members and generally perpendicular to the shafts. The elongated opening of the carrier is for aligning with the aperture of the base and an elongated slit of a collimator for passage-of an x-ray beam therethrough.
The assembly further includes a ball bearing sleeve positioned between the second support member and the non-threaded portion of the second shaft for allowing the second shaft to rotate with respect to the second member and for allowing the second member to slide on the non-threaded portion of the second shaft. A backlash-resistant nut assembly is threadingly received on the threaded portion of the second shaft and secured to the second support member.
The backlash-resistant nut assembly preferably comprises a first nut portion with a threaded bore, and a second nut portion having a threaded bore concentric with the threaded bore of the first nut portion, means for mutual engagement of the first and second nut portions, and means for biasing the first and second nut portions apart.
According to one aspect, the second shaft includes a second non-threaded portion separated from the first non-threaded portion by the threaded portion, and the carrier further includes a third support member received on the second non-threaded portion of the second shaft. The drive assembly further includes a ball bearing sleeve positioned- between the third support member and the second non-threaded portion of the second shaft for allowing the second shaft to rotate with respect to the third support member and for allowing the third support member to slide on the second non-threaded portion of the second shaft.
According to another aspect, the drive assembly further includes bearing outer sleeves positioned between the second and the third support members of the carrier and the ball bearing sleeves.
These and other objects and advantages of the invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the composition and apparatus possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure, the scope of which will be indicated in the claims.